1. Field of the Invention
This invention relates to an optical switch used for switching optical path lines in an optical communication system, and more particularly, to a semiconductor waveguide optical switch having a switching speed of the order of nanosecond.
2. Description of the Related Art
In an optical communication network utilizing optical fibers, the reliability and the economy thereof cannot be fully enhanced by simply connecting two distant places by means of the optical fibers. Therefore, in order to further enhance the reliability and economy, attempts have been made to improve the availability of the optical fibers by providing an optical switch or switches in the optical fibers to switch optical information to a standby line so as to detour obstacles or switch optical information to an unused line.
As the optical switch used in such an optical communication system, a mechanical type optical switch for switching the optical paths by mechanically moving the optical parts such as the optical fibers has been put into practical use. However, this type of optical switch has an inevitable problem that switching speed thereof is low and is of the order of millisecond (ms) and the number of switching times is limited by wear of the parts caused by the mechanical switching operations.
For the reasons described above, a semiconductor waveguide optical switch has been developed as an optical switch which theoretically has a switching speed of the order of nanosecond (ns) and is free from wear.
An optical switch having an X-junction optical waveguide shown in FIG. 1 is known in the prior art.
As shown in FIG. 1, thin semiconductor layers of a predetermined composition are sequentially laminated as a lower clad layer, a core layer and an upper clad layer on a semiconductor substrate 51 to form optical waveguides 52 and 53 in a ridge configuration. The optical waveguides 52 and 53 intersect each other in the shape of letter "X" with a branch angle .theta..degree. to form a junction point or branch point 54. The entire surface of the structure is covered with a thin insulation film.
That part of the thin insulation film which lies on the central portion of the branch point 54 is removed to form a narrow slit-like window (not shown) extending in a direction in which the optical waveguides are formed. For example, an adequate material is vapcuum evaporated on the upper clad layer via the window to form an electrode 55. The electrode 55 is used to inject a current of a predetermined value to the optical waveguides which intersect at the branch point 54.
Portions 52a and 53a of the optical waveguides 52 and 53 which lie on one side of the optical waveguides with respect to the branch point 54 constitute input ports, respectively, and the other side portions 52b and 53b thereof constitute output ports, respectively.
With the optical switch of the above construction, when a predetermined amount of current is injected via the electrode 55, the refractive index of that portion of the core layer which corresponds to the window and into which the current is injected is lowered by the action of the injected carriers. As a result, light waves incident on the input port 53a are subjected to total reflection at the interface between the current injection area and the non-injection area and then transmitted from the output port 52b to the exterior. On the other hand, when no current is injected via the electrode 55, light waves incident on the input port 53a straightly pass through the branch point 54 and are transmitted from the output port 53b to the exterior.
That is, the light waves incident on the input port 53a are transmitted out from the output port 52b or 53b depending on whether a current is injected via the electrode 55 or not. In this way, the optical switch of FIG. 1 performs the switching operation.
The current switching characteristic of the optical switch is shown in FIG. 2. FIG. 2 shows the output states of light from the output ports 52b and 53b when the current is injected via the electrode 55 while the light waves are incident to the input port 53a.
As is clearly seen from FIG. 2, the light outputs from the output ports 53b and 52b are respectively "1" and "0" when an injected current is 0. On the other hand, when the injected current is larger than a predetermined value (Isw in FIG. 2), the light outputs from the output ports 53b and 52b are changed to "0" and "1", respectively. That is, Isw is a threshold value for the light output. This type of optical switch is called a digital optical switch because of the nature of the response.
The injection current Isw may be influenced by the wavelength dependency of the optical switch. However, if the injection current is set to the maximum permissible value (Imax: Imax.gtoreq.Isw) which can be used in the operable condition of the optical switch, the optical switch will correctly perform the switching function of outputting "0" or "1" in all the operating conditions thereof according to whether the current Imax is injected via the electrode 55 or not. That is, when a current of Imax or more is injected, the wavelength dependency of this type optical switch can be eliminated.
This type of optical switch, that is, a digital switch, has the advantages over a waveguide optical switch utilizing the interference mode as will be described later that the switching operation can be attained simply by changing the refractive index of the optical waveguide according to the current injection and the wavelength dependency thereof can be eliminated. Further, it is possible to combine a plurality of the optical switches each having the X-junction optical waveguide so as to constitute an N.times.N exchange optical switch.
However, in order to operate this type of optical switch in an ideal manner, it is necessary to form the light reflection surface at exactly the central position of the branch point 54 at the time of current injection. In order to meet this requirement, it is necessary to form the slit-shaped window in exactly the right portion of the branch point 54 and form the slit with the precisely determined shape and dimensions.
However, at present, it is extremely difficult to form the slit-shaped window with such a high precision in the branch point 54 and the window will be formed in a position deviated in a right or left direction from the desired position of the branch point 54 although slightly.
With the deviation of the slit-shaped window in a right or left direction, the light reflection surface is accordingly deviated and therefore the optical switching characteristics will be degraded. In particular, in the case of a single mode device, the total width of the optical waveguide is approx. 10 .mu.m and therefore the deviation of the light reflection surface in a right or left direction develops into a serious problem.
Further, since the width of the slit-shaped window in the width direction of the optical path cannot be increased beyond a certain extent, the thickness of the light reflection surface portion formed by injecting a current via the window cannot be increased. As a result, light waves which should be fully reflected on the light reflection surface may pass through the light reflection surface, causing a problem that an excellent extinction ratio cannot be obtained.
A branching interference type modulator shown in FIG. 3 is known as another example of the optical switch. The modulator is constituted by a combination of Y-junction optical waveguides of the type shown in FIG. 4. As shown in FIG. 4, each of the Y-junction optical waveguides is constructed by sequentially laminating thin semiconductor layers of a predetermined composition as a lower clad layer, a core layer and an upper clad layer on a semiconductor substrate 61 to form an optical waveguide 62. The optical waveguide 62 includes a main optical waveguide 62a as an input port for light waves and two output optical waveguides 62b and 62c branching from the main optical waveguide 62 at a predetermined branch angle .theta..
Assume that the cross sections of the main optical waveguide 62a and the output optical waveguides 62b and 62c are the same. Then, the light waves incident on the main optical waveguide 62a are transmitted outwardly from the output optical waveguides 62b and 62c as light waves of the equal light outputs. More specifically, the light waves of the light output "1" incident on the main optical waveguide 62a are equally divided and then transmitted out from the output optical waveguides 62b and 62c as light waves of light output "0.5".
The construction of the branching interference type modulator constituted by a combination of the Y-junction optical waveguides is shown in FIG. 3. That is, the output optical waveguides 62b and 62c of one Y-junction optical waveguide are respectively connected to the input optical waveguides 62b' and 62c' of the other Y-junction optical waveguide, and electrodes 63a and 63b are respectively formed on the connecting portions of the waveguides. A predetermined voltage can be applied to the electrodes 63a and 63b. With the modulator, light waves incident on the main optical waveguide 62a are equally divided by the output optical waveguides 62b and 62c. In this case, for example, since the guided light propagating from the output optical waveguide 62c to the optical waveguide 62c' is subjected to the phase shift according to the voltage applied via the electrode 63a, the guided light is combined or interfered with the guided light propagating from the output optical waveguide 62b to the optical waveguide 62b'. As a result, the light output of the light wave transmitted out from the main optical waveguide 62a' varies according to the phase difference between the guided light propagating through the optical waveguide path 62c-62c' and the guided light propagating through the optical waveguide path 62b-62b'.
In the case of the branching interference type modulator, the mode interference of the light waves propagating through the optical paths is utilized. For this reason, the light output of the light waves to be transmitted is dependent on the polarization and wavelength of the light waves to be propagated. Accordingly, this type modulator can be properly operated only for the guided light of a specified polarization and a specified wavelength.
Besides the X-junction optical switch based on total internal reflection as shown in FIG. 1, another type of digital optical switch is also disclosed by Y. Silberberg, et al. in "Digital Optical Switch" in 11th Conference on Optical Fiber Communication (paper No. THA3). Their switch disclosed utilizes a lithium niobate waveguide as a substrate material, and its operation principle is based on "mode evolution". The mode evolution is the phenomenon that the light wave incident on the junction is transmitted only to the output optical waveguide whose propagation constant is larger than that of the other output optical waveguide. This phenomenon was first reported by H. Yajima in the article of Applied Physics Letters (vol. 12, pp. 647-649, 1973) "Dielectric Thin Film Optical Branching Waveguide" and it was applied to the optical modulation by W. K. Burns, et al. who wrote the article entitled "Active Branching Waveguide Modulator", pp. 790-792 of the volume 22 issue of Applied Physics Letters. Y. Silberberg, et al. used this phenomenon to achieve polarization and wavelength insensitive switching with a help of digital response.
The lithium niobate digital optical switch, however, has two main drawbacks. First, the device is large in length. This is because the linear electrooptic effect can induce a refractive index difference as small as 10.sup.-4. A typical electrode length is more than 10 mm. Secondly, a polarization independence is achieved at the cost of applied voltage. In the case of the lithium niobate, a polarization independent optical switch requires a voltage three times higher than that for a polarization dependent counterpart. This is because the linear electrooptic effect is anisotropic, that is, its magnitude depends on the direction of applied electric field and orientation of crystal.